The present invention relates to ring laser gyros and, more particularly, to a method of controlling such gyros according to a preselected cavity model to enhance gyro performance.
In a ring laser gyro, laser beams circulating in opposite directions around the same closed path are shifted in frequency when a cavity containing the path is rotated about a preselected input axis. With no angular motion about the input axis, the beams travel equal distances around the cavity and their optical frequencies are the same. Angular movement in either direction about the axis causes an apparent increase in cavity length for the beam traveling in the direction of movement and a corresponding decrease for the beam traveling in the opposite direction. Because the closed optical path is a resonant cavity providing sustained oscillation, the wavelengths of the beams increase and decrease, respectively. This causes a differential proportional to the angular rate of the input to occur between the beam frequencies. The frequencies heterodyne at a common photodetector, giving rise to a beat frequency directly proportional to the angular rotation rate.
At low angular rates, the frequency differential between the beams is small, causing the beams to resonate together or "lock-in" so that they oscillate at a single frequency. Thus, it is difficult to measure low angular rates because the frequency differential proportional to the angular rate does not exist. This problem is illustrated graphically in the upper portion of FIG. 4, wherein the gyro output signal (.psi.) is plotted against the angular rate of the gyro (.OMEGA..sub.in). In the graph, a diagonal line 10 bisecting the first and third quadrants represents the proportional output of a perfect gyro, while curves 11 represent the output of a real, undithered ring laser gyro having an output equal to zero at inputs between -.OMEGA..sub.1 and +.OMEGA..sub.1. The difference between the ideal and actual outputs is the bias error due to lock-in.
It is common practice to minimize bias error by mechanically oscillating or "dithering" the body of a gyro. Systems of this type are disclosed in U.S. Pat. Nos. 4,190,364, 4,314,174 and 4,344,706. They superimpose a relatively large dithering motion upon the gyro input, averaging the error so that low rates of input are detectable. At present, body dither is accomplished at frequencies on the order of 400 hertz and angular velocities of approximately 120 degrees per second. However, such movements are difficult to control and typically leave a significant level of bias error.
A variety of techniques have been proposed for further reducing the bias error of mechanically dithered gyros, the most common being to superimpose a secondary random dither which reduces error accumulation. However, such secondary dither significantly increases "random walk" and presents a practical lower limit on signals detectable by it.
In operating a ring laser gyro it is important to maintain cavity length constant, at a value corresponding to a preselected laser intensity within one of several "modes" of cavity operation. Selection of a mode is accomplished by applying a voltage to a transducer associated with a mirror of the cavity. Ring laser gyros have heretofore been constrained to operate within a single predetermined mode, chosen on the basis of lowest random walk at a typical operating temperature, and have relied upon high frequency modulation of the control mirror and a simple closed loop feedback system to track the laser intensity to the peak of the mode. The modulation frequency has typically been in the neighborhood of 6 kilohertz. In this context, prior gyros have operated under a rigid set of constraints which were believed necessary for satisfactory operation.
Therefore, it is desirable in many applications to provide an apparatus for controlling a ring laser gyro cavity in a manner which minimizes errors due to lock-in, random walk and other sources.